The present invention relates to activators provided in catalytic systems suitable for the polymerization of ethylene.
A large number of different Ziegler type catalysts are already known for the polymerization of ethylene and .alpha.-olefins. These catalysts generally comprise the catalyst component itself, consisting of at least one halogenated compound of a transition metal of Groups IV to V of the Periodic System, and an activator, selected from hydrides and organometallic compounds of metals of Groups I to III of the Periodic System. The catalyst component comprising the transition metal may be fixed, if so desired, on an inert support such as alumina, silica, magnesium oxide, magnesium halides, etc.
More particularly, the present invention relates to activators provided in Ziegler type catalytic systems, capable of polymerizing ethylene, optionally in the presence of at least one .alpha.-olefin, under high pressure and at high temperature. The invention also relates to a process for the polymerization of ethylene under elevated pressure, above 200 bars, and at elevated temperature, above 170.degree. C., in the presence of such catalytic systems.
The polymerization activators most frequently mentioned in the literature are the hydrides of lithium and/or aluminum, trialkylaluminums, alkylsiloxalanes, and alkylaluminum chlorides. The literature gives isolated examples of activators, containing a halogenated aluminum compound, the halogen of the compound being other than chlorine. However, French Patent Specification No. 1,214,965 describes the use for the polymerization of .alpha.-olefins, particularly those having from 3 to 10 carbon atoms, at low temperature, below 150.degree. C., and under low pressure, from 1 to 30 atmospheres, of an activator consisting of a mixture of alkyl-metal halide, the halogen being other than fluorine, and of alkyl-metal fluoride, the ratio of halide to fluoride lying between 9 and 1. French Patent Specification No. 1,255,970 describes the polymerization of olefins at low temperature, below 100.degree. C., in the presence of a catalyst, obtained by treating the reaction mixture of TiCl.sub.4 and a halogenated organoaluminum compound with a mixture of a trialkylaluminum and a metal fluoride, the ratio of trialkylaluminum:metal fluoride:alkylaluminum dihalide (formed by reduction of TiCl.sub.4) lying between 0.1:0.9:1 and 0.9:0.1:1. Finally, Japanese Patent Specification No. 71/34,612 describes the polymerization of propylene at 160.degree. C. and under 35 bars in solution in the presence of a mixture of titanium trichloride and diethylaluminum fluoride in a molecular ratio Al/Ti equal to 2.
On the other hand, it is known to polymerize ethylene under a pressure of between approximately 200 and 2,500 bars, and at a temperature of between approximately 160.degree. and 300.degree. C. Within the scope of such a process, the goal is to improve the quality of the polymer produced by modifying the following parameters in particular: density, molecular weight, and molecular weight distribution.